436 Mr. Robert Hunt on Chromo-Cyanotype^ 



John Herschel, which in many respects it resembles, I pro- 

 pose to call it chromo-cyanotype, the propriety of which 

 term will, from the following description of the process, be 

 sufficiently apparent. 



It is now well known that the bichromate of potash is very 

 susceptible of change under the influence of the chemical 

 principle of the solar rays, which, for reasons I have elsewhere 

 assigned*, I am disposed to consider as an independent prin- 

 ciple, and for which I have suggested the name Energia. At 

 the last meeting of the British Association, I published a pro- 

 cess (the chromatype) in which a mixture of the bichromate 

 of potash and the sulphate of copper was the photographic 

 material employed. In the process to which I would now call 

 attention, the bichromate is used in combination with the fer- 

 rocyanate of potash. I draw attention to this, more particu- 

 larly, to show how very varied are the photographic effects 

 produced by the combinations of chromic acid ; in addition to 

 those already published, my experience has brought me ac- 

 quainted with at least twenty others, which promise, with at- 

 tentive manipulation, to produce singularly varied and beauti- 

 ful results. 



To one ounce of a saturated solution of the bichromate of 

 potash add half an ounce of a solution of the ferrocyanate of 

 potash containing half a drachm of the salt. The solutions 

 upon mixing become a dark brown, but no precipitation takes 

 place. With this mixture wash over one side of a sheet of 

 letter paper and dry it by the fire. Upon paper thus pre- 

 pared a picture is impressed in the ordinary way, the image 

 being a very faint negative one. This paper is not sufficiently 

 sensitive to be affected by the subdued light of the camera 

 obscura, but in the sunshine very interesting copies of engra- 

 vings may be procured. Upon this paper, as upon the com- 

 bination of the bichromate of potash and the sulphate of copper 

 used in the chromatype process, the sun's rays exert two di- 

 stinct actions, browning the paper in the first instance, and 

 then with some rapidity bleaching it. Prismatic analysis 

 shows these two dissimilar effects to be produced by rays of 

 nearly the same degree of refrangibility. The first browning 

 takes place under the mean blue ray of the spectrum, and 

 gradually extends itself upwards to beyond the violet ray, and 

 downwards to the green ray ; and the first indication of whi- 

 tening is seen about the mean violet, and it extends slowly 

 over the whole of that space which has been previously dark- 

 ened. We have several curious instances in which rays are 

 seen to destroy their own work in this manner. 

 * See Researches on Light, by the author. 



